In the normal stable working range of an axial flow fan all vane channels between the fan blades are flown through equally and continuously by the air volume transported by the fan.
However, in the unstable working range a stall condition arises in the form of great pressure variations as a result of which the fan blades may be subjected to severe damage or even destruction when operating in this working range for some time. Therefore, operation in this unstable working range should be avoided and usually such working conditions for an axial flow fan are aimed at that the fan will operate in the stable working range only.
The pressure variations occurring in the unstable or stalling range express themselves by the creation of one or more zones in which the air flow through the vane channels is blocked or goes in the opposite direction and it has been shown that such stalling zones move around the fan wheel with an angular velocity smaller than the rotational speed of the wheel.
In an article by Hartwig Petermann in the German periodical "Stahl und Eisen", No. 9, Apr. 27, 1961 pages 589 to 593 and in German published patent application (DAS) No. 1,232,310 it has been suggested to utilize the latter recognition to detect the stall condition of an axial flow fan by measuring the total air pressure in a direction opposite to the direction of revolution of the fan wheel in a radial measuring plane immediately in front of the fan blades relative the atmospheric pressure by means of a hook-shaped tubular measuring probe. It has been shown that a curve representing this relative pressure as a function of the air volume transported by the fan has a characteristic knee at the junction between the stable working range and the stalling range in the fan characteristic so that such a measurement will be suitable for detecting the arise of the stall condition.
However, this known method suffers from the limitation that it is only applicable in connection with axial flow fans having stationary, i.e. non-adjustable vanes and rotating at a constant speed and in such cases where control members in the form of dampers or the like are not installed in the air duct on the suction side of the fan. Thus, if control dampers are present, the static pressure in the measuring plane and, as a result thereof, also the pressure value associated with said knee on the detector characteristic will unavoidably be changed so that said knee will no longer be detectable by a conventional pressostat and the known method will not be suitable for supervision and control of axial flow fans of this kind in order to prevent them from entering into the stall condition.